Electrets in the form of a resin film or fabric sheet are used in various fields of application. Film electrets are used in acoustic devices such as microphones, speakers and cartridges, in devices in measuring instruments such as vibration meters and strain gages, and in contactless switches. Attempts are being made to extend the use of those films to memory devices and the medical area. On the other hand, sheets of fiber electret are used in dust collecting filters in home air conditioners, air cleaners, vacuum cleaners, etc., and in filters in industrial air conditioning equipment.
While various methods are known for making electrets, such as thermal electret, electroelectret, radioelectret, magnetoelectret, photoelectret and mechanical electret forming methods, the first two are currently employed in practice to make electrets out of resin films and fiber sheets, principally from the viewpoints of performance and cost. The thermal electret forming method is capable of yielding electrets that carry stable surface charges. However, this method is based on a batch system in which the starting material is first heated to a temperature above its melting or softening point and then cooled with a high DC voltage being applied. This is a time-consuming process and is not highly productive. On the other hand, the electroelectret forming method is highly productive since electrets are formed by injecting electric charges with high dc voltage being applied to the starting material at room temperature. However, the surface charges on the electrets are not highly stable.
Further, the films electrets made by the said conventional methods, as well as the electret filters prepared from those films have necessity to improve retention of surface charges.